This invention relates generally to an index signal detecting system for detecting an index signal recorded in a track area separate from those track areas of a video signal and PCM (pulse code modulated) audio data so as to carry out a program search for use with a rotary head type recording and/or reproducing apparatus, such as an 8 mm video tape recorder (VTR) or the like.
The so-called 8 mm VTR (video tape recorder) can employ a recording mode for recording an audio signal which is frequency-modulated and mixed with a color video signal under the condition that the audio signal is separable from the color video signal in frequency. Also, the 8 mm VTR can employ an optional recording mode in which the audio signal is pulse-code-modulated and recorded in an area separate from that of the color video signal so as to form a single skewed track.
FIG. 1 schematically illustrates one example of a rotary head assembly used in the 8 mm VTR and FIG. 2 illustrates a tape format thereof.
Referring to FIG. 1, record and reproduce rotary magnetic heads HA and HB are arranged to differ from each other with respect to their recording gap angles, and are mounted on a rotary drum 1 with a rotational angle distance of 180.degree. therebetween. The rotary magnetic heads HA and HB are rotated at a frame frequency (30 Hz) in the direction by an arrow 3H under the state that they are slightly protruded from the periphery of the rotary drum 1. A magnetic tape 2 is wrapped around the periphery of the rotary drum 1 over an angular range of 221.degree. in a helical fashion and is transported at a constant speed during recording and normal playback in the direction shown by an arrow 3T.
Accordingly, as shown in FIG. 2, a signal is recorded in such a manner that tracks 4A and 4B having corresponding lengths of 221.degree. in rotary head rotation angle are alternately formed on the tape 2 by the rotary heads HA and HB. On an area AP of about 36.degree. in rotary head rotation angle (including a margin to record a PCM audio signal in the so-called after-recording mode and a guard band area) from a time point at which the rotary heads HA and HB start to scan the tracks 4A and 4B, there is recorded a pulse-code-modulated and time-base-compressed audio signal associated with one field period of the video signal. On a succeeding area AV of 180.degree. of rotary head rotation angle, there are recorded a color video signal, an FM (frequency-modulated) one field period audio signal, and a tracking pilot signal. The remaining area of 5.degree. in rotary head rotation angle is assigned as a head detaching area used when the rotary head HA or HB detaches from the tape 2.
As described above, the 8 mm VTR is capable of recording and/or reproducing the PCM audio signal. Thus, this feature of the 8 mm VTR attracts particular attention and such a technology has previously been proposed to use the color video signal recording area AV as the PCM audio signal recording area so that the 8 mm VTR can be used also as an exclusive apparatus for recording and/or reproducing a PCM audio signal (see U.S. Pat. No. 4,542,419, issued on Sept. 17, 1985, incorporated herein by reference).
The above-mentioned previously proposed technology effectively utilizes the fact that the area AV of 180.degree. in rotary head rotation angle, where the video signal and so on are recorded, is five times as long as the PCM area AP of 36.degree. in rotary head rotation angle. Then, the area AV is equally divided by 5 to form 5 divided segment track areas AP2 to AP6 shown by .circle.2 to .circle.6 , in addition to the original PCM audio signal track area AP1 shown by .circle.1 for each of the tracks 4A and 4B, as shown in FIG. 3. Then, the PCM audio signal of one channel, that is, the PCM audio signal of one field period with the compressed time base, is recorded in and/or reproduced from each of 6 segment track areas AP1 to AP6.
Accordingly, in this case the audio signal of one channel can be recorded and/or reproduced by the area unit so that the 6 channel audio signal can be recorded and/or reproduced, thus providing a recording time (capacity) 6 times as long as the prior art recording time (hereinafter this technique will be referred to as a multi-PCM mode).
In the multi-PCM mode, a PCM signal processor may be a signal processor capable of processing a signal of one channel, and which is used in the prior art 8 mm VTR because the PCM signal is recorded and/or reproduced at the unit of every segment track area.
By the way, the track format of the above mentioned 8 mm VTR will be described more fully with reference to FIG. 4. Referring to FIG. 4, from the right-hand side at which the rotary head begins to scan the magnetic tape 2, a top portion area of 5.degree. in rotary head rotation angle is assigned as a tracing start area 11. An area of the latter half of this tracing start area 11 and having 2.06.degree. (corresponding to 3H periods where H is the horizontal period) in a rotation angle of a rotary head is assigned as a preamble area 12 which becomes a clock run-in area synchronized with a PCM data succeeding thereto. Next to the preamble area 12, there is provided a PCM data recording area 13 of 26.32.degree. in rotary head rotation angle in which a time-base-compressed PCM audio signal is recorded. An area next to the PCM data recording area 13 and having 2.06.degree. (3H) in rotary head rotation angle is assigned as a postamble area 14 which is used as a margin to cope with a case where the recording position is displaced in the after-recording mode and so on. A next area having 2.62.degree. in rotary head rotation angle is assigned as a guard band area 15 between a video signal area and the PCM data area 13. Next to the guard band area 15, there is provided a one field video signal recording area 16 over 180.degree. of a rotary head rotation angle. Then, an area next to the video signal recording area 16 is assigned as a head detaching area 17 of 5.degree. in rotary head rotation angle.
FIG. 5 illustrates in greater detail a track format of the multi-PCM mode. A one segment track area for the PCM audio signal is exactly the same as the PCM audio area in the track format of the 8 mm VTR. Referring to FIG. 5, this track format of the multi-PCM system is formed of a tracing start area 21, a preamble area 22, a PCM data area 23, a postamble area 24, and a guard band area 25. This track format is assigned to each of the segment track areas AP1 to AP6.
The PCM data is recorded such that the data "1" and "0" are modulated and then recorded on the tape 2. In the 8 mm VTR, for example, the data "1" is frequency-modulated as a signal having a frequency of 5.8 MHz and then recorded, while the data "0" is frequency-modulated as a signal having a frequency of 2.9 MHz and then recorded. In the preamble area 12 or 22 and the postamble area 14 or 24, there is recorded all "1" data, that is, the signal having a frequency of 5.8 MHz.
As a method for carrying out the program search in the 8 mm video tape recorder mode and the multi-PCM mode, the assignee of the present application has previously proposed a technique in which an index signal is recorded, for example, in the postamble area 14 or 24 in the respective track formats as described above, and this index signal is used as the program search (see U.S. patent application, Ser. No. 838,626, filed on Mar. 11, 1986).
According to the previously proposed invention, the index signal can be recorded and/or reproduced by the rotary head so that it is unnecessary to provide an exclusive stationary head for the index signal. Furthermore, the index signal can easily be recorded and/or erased in the after-recording mode.
The above mentioned program search using the recorded index signal is frequently carried out in the so-called high speed playback mode in which the tape is transported at high speed. In this case, the rotary head HA and HB scan a plurality of recording tracks as shown in FIG. 6.
Since the rotary heads HA and HB differ from each other in the azimuth angle of the recording gap, in the high speed playback mode the respective rotary heads HA and HB pick up the signal only after either one of the tracks 4A and 4B. Therefore, reproduced outputs of both the heads HA and HB become as shown in FIGS. 7A and 7B. In FIGS. 7A to 7D, reference letter TA designates a scanning locus of the rotary head HA, and reference letter TB designates a scanning locus of the rotary head HB, respectively. The envelope waveforms of the outputs from the rotary heads HA and HB in the respective scanning loci TA and TB are illustrated in FIGS. 7A to 7D.
In the high speed playback mode, although the drum phase servo can be applied to the rotary heads HA and HB, the tracking servo is not applied to the recording track so that the scanning position of the rotary head relative to the recording track pattern is not determined. However, after the high speed playback is started and the tape is started running, the scanning phase of the rotary head relative to the recording track pattern is determined so that the envelope waveform is repeated at the period corresponding to the scanning phase.
FIG. 7A shows an example of an envelope waveform of the output from the rotary head at a certain scanning phase when the tape speed is an even multiple of the normal tape speed. FIG. 7B shows an example of an envelope waveform of the output from the rotary head at a certain scanning phase when the tape speed is an odd multiple of the normal tape speed, respectively. When the tape speed is exactly the integer multiple of the normal tape speed, as shown in FIGS. 7A and 7B, the envelope waveforms of the reproduced outputs from the respective rotary heads HA and HB become fixed.
On the other hand, if the tape speed is not the integer multiple of the normal tape speed but is added with an offset value .alpha. to thereby make the tape speed the integer multiple .+-..alpha. (0&lt;.alpha.&lt;1), the envelope waveform is repeated at a predetermined period. For example, if .alpha.=.+-.1/2, the envelope waveform becomes an envelope waveform which is repeated at every 2 rotations (4 scannings) of the rotary drum 1 as shown in FIG. 7C. Whereas, if .alpha.=.+-.1/4, the envelope waveform becomes envelope waveforms which are repeated at every 4 rotations (8 scannings) of the rotary drum 1 as shown in FIG. 7D.
When the envelope waveforms of both the rotary heads HA and HB become the same, such as shown in FIG. 7B in which the tape speed is the odd multiple of the normal tape speed, if the position at which the rotary head scans the index area is displaced to the position at which the rotary head scans the track having a different azimuth angle from that of the rotary head, the envelope output becomes substantially zero as shown by the hatched area representing the index signal in FIG. 7B. Thus, the index signal cannot be reproduced at all. Furthermore, when the tape speed is the even multiple of the normal tape speed (in FIG. 7A), if the hatched area in FIG. 7A is taken as the index area, the index signal cannot be picked up at all from the reproduced output of the rotary head HB.
This defect can be avoided if the tape speed is added with the particular offset value .alpha. as shown in FIGS. 7C and 7D. As shown by the hatched areas in FIGS. 7C and 7D, although the reproduced output becomes zero in the index area portion by a certain single scanning, a reproduced output which is more than a predetermined level can be obtained in this index area portion by other scanning.
However, in order to afford the offset value .alpha., it becomes necessary to move the tape by the capstan in the high speed playback mode. Also, the higher the tape speed becomes due to a tape slip or the like, the more difficult becomes the tape speed control.
With a tape speed which is an integer multiple of the normal tape speed, there is another method for preventing the output reproduced from the index area by the rotary head from becoming zero. That is, the scanning phase may be controlled such that the output from the index area does not become zero. In that case, when the reel motor is servo-controlled or the tape is transported by the capstan, the capstan motor must be servo-controlled. Furthermore, it is very difficult to apply this method to the multi-PCM mode.